Alien Swarm is out now on Steam. And it's free.

Alien Swarm, Valve's free top-down xeno-blaster is now available to download from Steam . There is absolutely no reason for you to not download it and try it out immediately. It's like aliens, crossed with some guy yelling "OH GOD OH GOD I'M GOING TO DIE" down Teamspeak. We'll have an official PC Gamer server up and running ASAP, and will update the details in the comments below.

Just be aware that half the internet is trying to download it from Steam right now. So you might have to keep trying until you get a decent content server connection.